(a) Technical Field
The present disclosure relates to a gas diffusion layer for a fuel cell with improved operational stability.
(b) Background Art
One of the electrolyte membrane for use in a fuel cell vehicle is a polymer electrolyte membrane fuel cell (PEMFC). The PEMFC needs to stably operate over a wide current density range such that it may exhibit a high-power performance of at least several tens of kW under various operational conditions of the vehicle [S. Park, J. Lee, and B. N. Popov, J. Power Sources, 177, 457 (2008)].
A fuel cell generates electricity through an electrochemical reaction between hydrogen and oxygen. Hydrogen supplied to an anode (oxidation electrode) of the fuel cell is dissociated into hydrogen ions and electrons. The hydrogen ions are transmitted to a cathode (reduction electrode) through a polymer electrolyte membrane, and the electrons are transmitted to the cathode through an external circuit. At the cathode, the hydrogen ions and electrons react with oxygen (air) to generate electricity and heat and, at the same time, produce water as a by-product.
If the water is produced in an appropriate amount during the electrochemical reaction, it may function to maintain the humidity of the membrane electrode assembly (MEA). On the other hand, if the water is produced excessively and is not appropriately removed, flooding occurs at high current density, preventing the reactant gases from being sufficiently supplied into the fuel cell and thereby increasing voltage loss.
For example, especially when high power is required during operation of the fuel cell vehicle, if the reactant gases are not sufficiently supplied to the fuel cell by the flooding phenomenon, it is difficult for the vehicle to be stably operated. Accordingly, it is important to properly remove water produced by the electrochemical reaction of the fuel cell.
FIG. 1 is a schematic diagram showing a configuration of a unit cell including gas diffusion layers.
A gas diffusion layer (GDL) 220 is attached to the outer surface of the catalyst layer 110 coated on a fuel electrode and another GDL 220 is attached to the outer surface of the catalyst layer 110 coated on an air electrode. A polymer electrolyte membrane 100 is disposed between the catalyst layers 100. The gas diffusion layers 220 function to supply reactant gases such as hydrogen and air (oxygen), transmit electrons produced by the electrochemical reaction, and discharge water produced by the reaction to minimize the flooding phenomenon in the fuel cell.
Commercially available gas diffusion layers have a dual layer structure including a microporous layer (MPL) 200 having a pore size of less than 1 μm when measured by mercury intrusion and a macroporous substrate (or backing) 210 having a pore size of 1 to 300 μm [X. L. Wang, H. M. Zhang, J. L. Zhang, H. F. Xu, Z. Q. Tian, J. Chen, H. X. Zhong, Y. M. Liang, B. L. Yi, Electrochimica Acta, 51, 4909 (2006)].
The performance of the fuel cell depends on various characteristics of the GDL including thickness, gas permeability, compressibility, degree of hydrophobicity, structure of carbon fiber, porosity/pore distribution, pore tortuosity, electrical resistance, and bending stiffness (Japanese Patent No. 3331703 B2).
The microporous layer 200 of the gas diffusion layer 220 is formed by mixing carbon powder such as carbon black, acetylene black carbon, and black pearl carbon with polytetrafluoroethylene (PTFE) hydrophobic agent and coating the mixture on one or both sides of the macroporous substrate 210.
If the pore structure and the hydrophobicity of the microporous layer 200 is properly controlled, it is possible to efficiently discharge the water produced by the electrochemical reaction of the fuel cell, smoothly supply the reactant gases, and minimize the electrical contact resistance with the catalyst layer 110.
Meanwhile, the macroporous substrate 210 of the gas diffusion layer 220 is generally composed of carbon fiber and PTFE hydrophobic agent and may be formed of carbon fiber cloth, carbon fiber felt, and carbon fiber paper [S. Escribano, J. Blachot, J. Etheve, A. Morin, R. Mosdale, J. Power Sources, 156, 8 (2006); M. F. Mathias, J. Roth, J. Fleming, and W. Lehnert, Handbook of Fuel Cells-Fundamentals, Technology and Applications, Vol. 3, Ch. 42, John Wiley & Sons (2003)].
The macroporous substrate 210 of the gas diffusion layer 220 is used as a physical support for the polymer electrolyte membrane 100 and the catalyst layer 110. It, like the microporous layer 200, plays an important role in the mass transport for the electrochemical reaction. Especially, the macroporous substrate 210 plays an important role in discharging the product water to prevent the flooding problem.
It is necessary to optimize the structural design of the gas diffusion layer for the fuel cell such that it provides high performance in various application field and operational conditions. In general, a carbon fiber, cloth fiber felt or carbon fiber is used to prepare such gas diffusion layer.
Meanwhile, since a large number of components are disposed in a limited space of a fuel cell vehicle, it is important that the components are miniaturized.
A conventional gas diffusion layer is difficult to provide a stable operation under abnormal operational conditions such as a shortage in hydrogen supply. Also, since it is thick, the degree of freedom of vehicle design is affected.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.